Method and apparatus for sputter coating with variable target to substrate spacing

ABSTRACT

Thickness uniformity of films sputtered from a target onto a series of substrates is maintained as the target surface shape changes due to the consumption of the target. The eroded condition of the target is sensed by directly measuring the position of a point on the target surface, by measuring power consumption of the target, by measuring deposition from the surface of the target or by some other means. A controller responds to the measurement by moving a substrate holder to determine an amount to change the distance between the substrate and the target, usually by moving the substrate closer to the target, by an amount necessary to maintain uniformity of the coatings on the wafers being processed. A servo or stepper motor responds to a signal from the controller to move the substrate holder in accordance with the determined amount of distance change required. The adjustment is made following the coating of wafers at various times over the life of the target.

FIELD OF INVENTION

This invention relates to cathode sputter deposition and moreparticularly to a method and apparatus for overcoming the adverseeffects on deposited thin film uniformity from wafer to wafer due tochanges in sputtering target geometry as a result of the utilization oftarget material.

BACKGROUND OF THE INVENTION

In sputter deposition processes, substrates are placed in a processingchamber adjacent to a sputtering cathode target, which serves as asource of coating material. The pressure in the processing chamber,which is usually filled with an inert gas such as argon, is then reducedto a near vacuum, and a negative voltage is applied to the target. Thenegatively charged target emits electrons, which strike and ionize atomsof the gas to produce a plasma discharge. Often the plasma isintensified and confined over the target surface by the application of amagnetic field generated by magnets, which are usually placed behind oraround the periphery of the target. The large quantities of positiveions from the plasma that are produced in the sparse gas within thechamber are attracted to the negatively charged target, bombarding itssurface and thereby dislodging atoms or small particles of the materialof which the target is made from the surface of the target. The atoms orparticles move across the space in front of the target until they strikethe surface such as the surface of a semiconductor wafer or othersubstrate disposed, for example, in a plane parallel to the surface ofthe target, where they adhere to the substrate surface and form a thinfilm or coating layer thereon.

A primary consideration in designing a sputter deposition process haslong been to achieve a specified degree of uniformity in the thicknessof the resultant film being deposited on the substrate. In semiconductorwafer manufacturing processes, for example, such uniformities in thearea of +/−2 to 5 percent or better are currently being demanded.Factors that influence the degree of uniformity achieved in sputterdeposition include the relative sizes of the target and substrates, theconfigurations of field producing magnets and other factors controllingthe utilization or erosion profile of the target and the sputteringtarget to substrate spacing.

In the prior art, the factors of target to substrate size ratio, magnetdesign to control target erosion profile and target to substrate spacingare designed into the sputtering target and cathode assembly of thesputtering apparatus in an effort to produce the required film thicknessuniformity. For a given target material, and with other processconditions being held constant, cathode assembly design has provided anability to deposit films to some degree of the desired uniformities withtargets of limited thickness, where the erosion of the target surfaceover the life of the target cannot materially alter the target tosubstrate spacing that was the basis for the system design. With suchconstant geometries, those skilled in the art of sputtering systemdesign have concentrated on the control of erosion profiles, for exampleby altering magnet configuration, to fine tune the cathode design toachieve the desired film uniformity.

Typical prior art semiconductor wafer sputter deposition systems haveemployed targets of, for example ten inches (250 mm) in diameter toapply coatings to six inch (150 mm) diameter wafers. With suchapplications, uniformity in film thickness was approached by configuringsputtering cathode magnets to produce a greater sputtering rate aroundthe periphery of the target, usually outside of the six inch diameter ofthe wafer, to simulate the incidence of sputtered material onto thesubstrate from the remote regions of a sputtering target of infinitediameter, which in theory would produce a equal incidence of sputteredmaterial on every increment of the surface of the substrate. Theincreased sputtering rate around the periphery of the target compensatesfor target size limitations and increases the uniformity of thedeposited film.

Theoretically also, with the target of infinite diameter, uniformity ofthe deposited coating is not generally affected by target to substratespacing, at least not by spacing variations of thirty to fifty percentwhere other effects, not necessary to consider here, would not befactors. However, with finite targets, increased sputtering around aperipheral area of the target causes a more deeply eroded peripheralarea or annular groove to form around the rim of the target. As thetarget erodes, the target surface recedes from the substrate, and doesso faster at the target rim than at the center of the target. The targetto substrate spacing change produces substantial changes in the rate atwhich material is deposited in the vicinity of the rim of the substrate.However, at the center of the substrate, the deposition rate is muchless affected by such changes.

With a ten inch target used for coating six inch wafers, targets havingthicknesses of from one-sixteenth inch to one and one-half inches arecommonly found. Typically, target-to-substrate spacing with such targetsmay be approximately two inches. With the thin target, thetarget-to-substrate spacing change experienced over the life of thetarget will be at most about three percent, which should have anegligible effect on the deposition uniformity on the substrate. Withthe thicker targets, however, the erosion of a peripheral groove canresult in an increase in target-to-substrate spacing, at certain pointson the target, by more than seventy percent. Such changes can result insubstantial decreases in the deposition rates on the substrate,particularly in the vicinity of the substrate rim. Thus, cathodesdesigned to produce a desired coating uniformity on wafers early in thelife of the target do not coat wafers with sufficiently uniform filmslate in the life of the target.

Some prior art systems have been proposed in which the deposition rate“roll-off” or decrease over the life of the sputtering target due to theprogressive erosion of the target is offset by an increase in sputteringpower. Such increases in many such systems have a uniform compensatingeffect across the surface of the target. Thus, where the erosion rateroll-off is usually greater at the peripheral groove on the target thanat the target center, the uniformity of the coating changes along withthe reduction in deposition rate as the target erodes, while the loss ofuniformity is retained as the sputtering power is increased. Somesystems have disproportionately increased sputtering power around theperipheral groove. While such adjustment is possible where stationaryelectromagnets are used, in those sputtering systems where rotatingpermanent magnets are desired, magnet field compensation for non-uniformdeposition rate roll-off is less practical and less effective. Thisincreases cathode assembly complexity, and is difficult with one piecesputtering targets, and tends to even more greatly increase the erosionrate around the rim of the target in proportion to the area in thetarget center. Such schemes of compensating for erosion, however, alsohave effects on voltage levels, component heating and plasma shapingthat have other often adverse, undesirable or troublesome effects.

Accordingly, there is a need for an effective and efficient method andapparatus for maintaining high degrees of deposited film uniformity in asputter coating process, particularly where thick targets are employedwhich substantially are substantially eroded over their useful lives.

SUMMARY OF THE INVENTION

It is a primary objective of the present invention to maintain a desiredsputtered film thickness distribution throughout the life of asputtering target. A more particular objective of the present inventionis to overcome changes in deposited film uniformity caused by changes inthe geometry of the surface of a sputtering target as the target erodes.A specific objective of the present invention is to provide a method andapparatus for maintaining the uniformity of films deposited by a thicksputtering target, from substrate to substrate, over the lifetime of thetarget. It is a further objective of the present invention to providefor the automatic adjustment of conditions or parameters of sputterdeposition to consistently produce uniform film thickness on substratesbeing coated, over the useful life of the sputtering target, without theneed for manual adjustment by an operator or other operatorintervention.

In accordance with the principles of the present invention, there areprovided a method and an apparatus in which the relative position of thesubstrate in relation to the position of the sputtering target ischanged to compensate for changes in target geometry, due to theconsumption or erosion of the surface of the sputtering target duringuse, and to thereby maintain the deposition uniformity. The change intarget-to-substrate spacing is preferably made automatically in responseto a measurement of some parameter related the target erosion, and ismade to achieve a distance that varies in a predetermined relationshipor as a predetermined function of the state of erosion of the target.

According to the preferred embodiment of the present invention, therelative positions of a substrate holder and a target support on acathode assembly are changed during the life of the target. The positionchanges are made to maintain a predetermined distance between thesurfaces of substrates that are to be coated, when mounted on thesubstrate holder, and the surface of a sputtering target, supported onthe cathode holder in the cathode assembly, as the target is consumedduring the processing of a sequence of wafers. As a first order ofapproximation, the predetermined distance, as a function of targeterosion, may be considered a linear function that maintains thesubstrate at a constant distance from the average depth of the erosiongroove. However, due to effects such as self-shadowing, redeposition,and other effects, it is generally found that the function is anincreasing function requiring a greater decrease in target-to-substratespacing for each unit of erosion of the peripheral groove. Furthermore,the relationship between the relative positioning of the substrate andtarget and target erosion is dependent on target material, magnetrondesign, the absolute distance from target to substrate and otherfactors. Thus, either a correction table, a list of coefficients or aspecific function is preferably provided, most conveniently in softwareor in the form of data to be input to a controller of the machine. Suchdata or function may be generated, for example by a sputtering apparatusand target manufacturer, by coating test wafers, one at each of aplurality of spacings at, for example, 0.05 inch increments over arange, at various times, for example 10 or more, over the life of a testtarget. In this way, a table or equation may be generated describing therelationship between target-to-substrate spacing and coating uniformity.

The spacing of substrates supported on a holder in a sputter coatingchamber and a target of a sputtering cathode assembly in such chamber isso maintained throughout the sputtering life of the target, bycontinuous or periodic adjustments of the target to substrate spacing.Normally this requires reducing the distance between the target andsubstrate holders, but in some cases an increase in the distance may berequired to maintain uniformity. The adjustments may be made during theprocessing of individual substrates but are preferably performed betweencoating operations, when changes of the wafers or substrates are beingmade. Such distance changes are made progressively at various intervalsthroughout the life of the target, whenever enough of a change in theprofile of the target is anticipated to affect coating uniformity. Suchchanges might be made after every fifty wafers are processed, or at onehundred or more times over the life of a target. With thicker targetssuch as aluminum which deposit thicker films onto the substrates andaccordingly erode to depths of more than an inch over their lifetimes,more adjustments are required than with less thick targets.

In the preferred embodiment of the invention, a substrate holder ispositioned in a sputtering chamber at an initial distance form asputtering cathode assembly when a target is new and uneroded so as toachieve a designed target-to-substrate spacing. As the target erodes andits surface recedes from its initial position, the substrate holder iscaused to move toward the target to maintain a predeterminedtarget-to-substrate relationship. As the target erodes more deeply, thepredetermined relationship changes in accordance with the collected datato maintain coating uniformity.

In the preferred embodiment of the invention, a thick target is employedwhich, during its lifetime, substantially changes in shape as materialfrom its sputtering surface is depleted. Such targets are, for thecoating of semiconductor wafers, for example, frequently circular inshape and somewhat larger in diameter than the diameters of circularsubstrate wafers being coated. Further, such targets frequently areprovided with plasma shaping and enhancing magnetron structures thatcause the sputtering to occur at a greater rate from a peripheral ringor annular erosion groove, usually lying outside of the diameter of thewafers being coated, than from the circular central region of the targetlying within the annular groove. This use of a peripheral erosion groovecauses the surface of the target to recede from its initial positionfaster at the area near the target rim than at the central area. In thepreferred embodiment of the invention, the target-to-substrate spacingis readjusted, as wafers are processed over the life of the target, tomaintain a target-to-substrate distance between the surface of thesubstrate and the average sputtering surface of the target to achievedesired coating uniformity.

Further in accordance with certain preferred embodiments of theinvention, there is provided a sputtering apparatus having thecapability of determining the state of erosion of the target andparticularly the depth of the average of the erosion groove on thetarget surface. Such determination may be made by direct surfaceposition measurements, for example with one or more sensors provided inthe chamber. Such sensors could include, for example, laser opticaldevices that use light reflected from points on the target surface andproduce signals related to the position or distance of the point on thetarget surface to the sensor. Due to target surface irregularities thatdevelop as a target is eroded, a scanning type laser that takes severalmeasurements and/or logic in a computer or controller to interpret themeasurements would be required.

Preferably, however, erosion measurement is made by an alternativemethod of measuring the power consumed by the target over each incrementof time from the beginning of the life of the target and themeasurements summed so as to integrate the power measurement up to thepresent. This produces a value representative of the total energyconsumed by the target from the beginning of its life to the present.This energy data is stored and continually updated. Such total energyconsumption is correlated with stored data to determine the specificerosion state of the target. This is further correlated with stored datato produce a calculated representation of the erosion profile as afunction of the age of the target measured in units of consumed totalenergy. The measurement of energy need not be from the absolutebeginning of the life of the target but may be from any beginning pointover the life of the target, with data being stored relating to the useof the target or its surface condition up to the beginning point of theenergy measurements.

Further in the alternative, other forms of target erosion measurementmay be used, such as by counting the number of wafers coated andcorrelating the count with data relating the count to target usage, in amanner similar to that for the measurement of energy discussed above.

In response to such measurements of target erosion, the spacing betweenthe target holder and substrate support is adjusted, usually by reducingsuch spacing to achieve or maintain a desired distance between thetarget surface and substrate surface, preferably by moving the substratetoward the target. The amount of adjustment is preferably based on adetermination from the measurements of the change in the depth of themost deeply eroded portion of the target, particularly where thatportion is a peripheral groove. Alternatively, the amount of adjustmentcan be keyed primarily to the amount of erosion of the peripheral regionof the target, or to some other function of erosion that has beenempirically or by computer modeling determined to have a known relationto film uniformity, so that the amount of adjustment can be calculatedbased on the determination. In the embodiment of the invention in whicha target is eroded to form a peripheral erosion groove, the adjustmentis made by moving the substrate somewhat less than would be required tomaintain a constant distance between the substrate surface and thebottom of the peripheral groove. Preferably, the sputtering apparatus isprovided with a microprocessor in its control that calculates suchadjustment based on a stored table or function in memory.

In response to the calculations based on the target surface positionmeasurements, either the target or substrate, but preferably thesubstrate, is moved to adjust the target-to-substrate spacing, keepingthe target and substrate parallel. The movement may be achieved byproviding a servo or stepper motor linked to the substrate holder, forexample, by a gear or ball screw drive or other suitable drivemechanism. Preferably, a feedback sensor of some sort is employed toverify the operation of the motor. Such, sensor may be a positionindicator built into the servo or stepper motor or motor driver, or suchas a position decoder or resolver linked to the motor output orotherwise between the substrate and substrate support.

In an alternative embodiment, a deposition rate sensor may be employedto measure the incidence of sputtering material across the wafersurface. Such a sensor may be set up to take deposition readings betweenthe processing of wafers so that the sensors can more easily occupy theposition of the wafer. The controller of the apparatus may then beprogrammed to adjust the position of the substrate holder or targetuntil uniform incidence of material is realized. In the alternative,servo or stepper motor with feedback control from such a sensor may beused.

With the method and apparatus of the present invention, the sputteredfilm uniformity that existed across the surfaces of wafers sputteredfrom the target at the beginning of the target's life is closelymaintained across wafers coated sputtered throughout and toward the endof the life of the target.

These and other objectives of the present invention will be more readilyapparent from the following detailed description of the preferredembodiment of the invention in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagrammatic representation of a sputter coating apparatusaccording to the prior art.

FIG. 2 is a cross-sectional diagram of the sputtering target of theprior art FIG. 1 showing the initial position and shape of itssputtering surface and two erosion profiles that are typical at pointsin the life of the target, and associated target-to-substrate distances.

FIG. 3 is a graph illustrating the effects of changes intarget-to-substrate distance on sputtered film uniformity.

FIG. 4 is a graph similar to that of FIG. 3 but illustrating the changesin sputtered film uniformity with the prior art apparatus of FIG. 1 atthe surface position and erosion profiles shown in FIG. 2.

FIG. 5 is diagrammatic representation of a sputter coating apparatusaccording to principles of the present invention.

FIG. 6 is a cross-sectional diagram similar to FIG. 2 of the sputteringtarget of the apparatus of FIG. 5 showing the initial position and shapeof its sputtering surface and two erosion profiles that are typical atpoints in the life of the target and associated target-to-substratedistances.

FIG. 7 is a graph similar to that of FIG. 4 illustrating the changes insputtered film uniformity with the apparatus of FIG. 5 at the surfaceposition and erosion profiles shown in FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A typical sputtering apparatus 10 of the prior art is diagrammaticallyillustrated in FIG. 1. The apparatus 10 includes a vacuum processingchamber 11 in which is mounted a wafer or other substrate holder orplatform 12. The platform 12 is typically situated and oriented parallelto the surface 13 of a sputtering target 14 held in a sputtering cathodeassembly 15. The platform 12 may be moveable into and out of the chamber11 but is usually maintained in a fixed position, at least relative tothe cathode assembly 15, during the sputter deposition of films ontowafers, such as wafer 16, held by clamping structure (not shown) on thesurface of the platform 12. A common configuration of such prior artsputtering apparatus 10 provides the cathode assembly 15 with a magnetassembly 17, which produces a magnetic field (not shown) over thesurface of the target 14 that confines and enhances a plasma adjacentthe surface 13 of the target 14. Such magnet assembly 17 is frequentlypositioned behind the target 14, as illustrated, or may be positioned atleast in part around the periphery of the target 14. Such magnetassembly 17 may be stationery or moveable, and may be formed ofpermanent magnets or electromagnets.

The cathode assembly 15 of such an apparatus 10 is usually electricallyinsulated from the wall of the chamber 11, for example by mounting thecathode assembly 15 to the wall of the chamber 11 through an insulatorring 18. The cathode assembly 15 is energized to a negative potentialrelative to the wall of the chamber 11 by a power supply 19.

Sputtering systems of the prior art, such as the apparatus 10, aredesigned to provide a desired coating film distribution across thesurface of the substrate 16. Such design will include the provision ofan optimal target-to-substrate spacing and a specifically designedmagnet configuration. The drawings illustrate a typical configuration inwhich the target 14 has a diameter greater than that of the substrate 16to be coated. For example, for substrates in the form of circular wafershaving a diameter of about six inches, a target 14 typically is circularand has a diameter of about ten inches. With such targets 14, thecathode assembly 15 typically includes a circular magnet array behindthe target 14, often in the form of a rotating magnet having an axis atthe center of the target, which produces a increased magnetic fielddwell around the rim of the target, causing enhanced emission ofsputtered material around the rim of the target 14 and outside of therim of the wafer or substrate 16, thereby causing the formation of aperipheral erosion groove as the target 14 is used. The erosion profileis achieved by the design of the magnet assembly 17, which, once aprofile is determined, is within the skill of those engaged in thedesign of such magnet assemblies in the sputtering art.

A typical erosion profile is illustrated in FIG. 2, in which the target14 is shown with its initial surface 13 spaced a distance S₁ from thesubstrate 16. The surface 13 represents the location of the targetsurface within, for example, the first 50 kilowatt hours (kWH) of usageof the target 14. At this stage in the life of the target, the profileconforms closely to the original surface of the target. While aperipheral groove tends to form at this stage, its depth is not sosignificant as to have caused a change in the uniformity of depositedfilms from wafer to wafer. For thin targets, this remains the casethroughout the life of the target.

In FIG. 2 is also illustrated an erosion profile 22, which is typical ofan eroded shape of the surface of the target 14 after, for example, 750kWH of usage where the target 14 is relatively thick. Such a profiledevelops an erosion groove 23 adjacent the periphery of the target 14that significantly changes the distance between some parts of the targetsurface and the substrate. The bottom of the groove 23 is located adistance S₂ from the substrate 16. Similarly, also illustrated inphantom lines is an erosion profile 24, which represents the erodedshape of the surface of the target 14 after, for example, 1500 kWH ofusage. Such a profile has its erosion groove 25 adjacent the peripheryof the target 14 a distance S₃ from the substrate 16. Targets that takeon this form are relatively thick targets, such as those of aluminum,that are used to sputter blanket base conductor layers of, for example,one micron in thickness on a wafer. Over the life of such a target,where several thousand wafers are so coated, a peripheral groove of oneinch or more in maximum depth can be formed. The affects of such erosionon the uniformity of the deposited coatings is demonstrated by FIGS. 2and 4.

With a new target 14 having its surface essentially conforming to thatof surface 13 of FIG. 2, and with an optimized target-to-substratespacing of, for example two inches, the sputtered film thicknessdeposited on the wafer 16 may be that represented by the curve 30 inFIG. 3, which is plotted in arbitrary units across the surface of a sixinch wafer. The uniformity of the thickness represented by the curve 30is, for example, +/−2% across the surface of the substrate 16.

Also illustrated in FIG. 3 is a curve 31 representing the thickness ofthe deposited film across the surface of the wafer with the spacing fromthe surface 13 to the wafer 16 reduced to 1.75 inches. In such aconfiguration, a uniformity of +/−3.7% is realized across the surface ofthe six inch wafer. Such degradation in uniformity is typical ofchanging the target-to-substrate spacing of a system that has beendesigned for 2.0 inches to 1.75 inches. Similarly, curve 32 representsan erosion profile having a uniformity of +/−5.8%, and is typical ofthat resulting from a change in the target-to-substrate spacing from adesigned spacing of 2.0 inches to 2.25 inches. As can be seen, theeffects of changes in the target-to-substrate spacing are morepronounced around the periphery of the substrate 16 but are not muchless pronounced and almost insignificant near the center of thesubstrate 16. It is the change in distance and angle of incidencebetween the substrate 16 and the erosion groove 23 and 25, resultingfrom a deviation from the designed spacing, that causes this effect.

In to FIG. 4, the curve 30 is again illustrated. With a new target 14that is within the first 50 kWH of usage, the uniformity of the filmdeposited onto wafers 16 therewith is, for example, 2.0%. Alsoillustrated are two curves 33 and 34 representing film thickness, inarbitrary units, across the surface of a six inch wafer, with thetarget-to-substrate spacing maintained at the initial spacing S₁ equalto 2.00 inches from the initial surface 13 of the target 14. Curve 33represents the film thickness distribution across the substrate 16 after750 kWH of usage of the target and corresponds to the uniformitydistribution on wafers coated with the target 14 at the state of erosionillustrated by the erosion profile 22 of FIG. 2, at which point theuniformity has deteriorated to 3.1%. Curve 34 represents the filmthickness distribution across the surface of the substrate after 1500kWH of usage of the target 14 and corresponds to the uniformitydistribution on wafers coated with the target 14 at the state of erosionillustrated by the erosion profile 24 of FIG. 2, at which point theuniformity across wafers has further deteriorated to 6.2% across thewafer.

FIG. 5 illustrates diagrammatically one preferred embodiment of asputtering apparatus 40 according to principles of the presentinvention, which is similar to the apparatus 10 of FIG. 1, but differsas set forth here. In the apparatus 40, the support platform 12 on whichis mounted the wafer 16 is movably mounted in the sputtering chamber 11through a flexible sealing bellows 42, which is sealed to a supportingstem 43 on the bottom of the platform 12. The stem 43 is mounted so asto be driven toward and away from the cathode assembly 15 on a platformdrive assembly 44, which may be in the form of a ball screw drive or, asillustrated, include a screw drive 45 that is driven by a servo orstepper motor 46 through a set of gears 47 and 48. Any such drivemechanism that can move the substrate with respect to the target withprecision, without interfering with the sputtering process, may be used.

The operation of the motor 46 is controlled by a computer or logiccontroller 50, which delivers a control signal to the motor 46 inresponse to which the motor 46 drives the platform 12 to a specifiedposition relative to the cathode assembly 15. The position of theplatform 12 is monitored by a sensor 51 that generates a feed backsignal to the controller 50. The sensor 51 is, for example, a linearresolver, as illustrated, or may be provided by feedback elements thatare provided with the motor 46. The controller 50 generates the controlsignal to the motor 46 in response to a determination of the erodedcondition of the target 14. The determination may be made in response todirect measurement by a non-contact sensor of the target 14. Preferably,however, in the alternative or in addition, the controller may correlatea measurement of the total energy consumed in the sputtering processsince the target 14 was first installed in the machine 40. Suchinformation may be derived from a power measurement or cumulative energymeasurement on output 55 from the power supply 19. Such energy metersare typically provided on power supplies such as supply 19 that arespecifically designed for use in sputter coating equipment. Totalsputtering energy consumed by a target has a direct relation to thesputtering profile of a target with a given cathode configuration andtarget type. Thus, empirical data stored in the computer 50 provides abasis for looking up or calculating the erosion profile or depth of theperipheral erosion groove at any point in the life of the target 14based on the measured energy consumed by the target 14 over its life.

The operation of the controller 50, which preferably contains digitalmemory and a specially programmed microprocessor for such purpose,generates the control signal to the motor 46 so as to progressivelyadjust the spacing of the wafer 16 from the target 14 as illustrated inFIG. 6, over the life of the target.

The result of the invention in which the apparatus 40 of FIG. 5 isoperated as illustrated in FIG. 6, is the series of deposition filmthickness curves illustrated in FIG. 7, in which the curve 30 is thesame as that of FIGS. 3 and 4, with target life being not more than 50kWH with the spacing of the substrate 16 being 2.00 inches from theinitial position of the surface 13 of the target 14, at which theuniformity is +/−2.0%. Curve 57 represents the film across a wafer 16coated with the target having 750 kWH of usage, and with the spacing ofthe wafer 16 from the target being reduced such that the distance S₄ is1.85 inches by moving the platform 12 closer to the cathode assembly 15through operation of the motor 46. The substrate 16 is thereby moved0.15 inches. The uniformity of the coating on such wafer is +/−2.1%.

As FIG. 7 further illustrates, curve 58 represents a uniformity of 2.3%across a wafer 16 coated with the target 14 at 1500 kWH of usage on thetarget. This is realized by moving the substrate 14 to a distance S₅ of1.65 inches from the bottom of the groove 25 of erosion profile curve 24of FIG. 6 This is achieved by the motor 46 moving the platform 12 duringthe first 1500 kWH of usage of the target 14 a total distance that is0.35 inches closer to the target 14 than it was when the target 14 wasnew.

In operation, the position of the platform 12 is preferably periodicallyadjusted during the life of the target 14. With targets of thicknessesof, 1½ inches, a large number of, for example, several thousand wafersare coated with a single target 14 before the target 14 is completelyexpended. Such adjustment of the position of the platform 12 may then bemade after, but preferably not during, the processing of each wafer, orafter the processing of any predetermined number of wafers. Wafer countcan be used to determine the amount of movement to be made to theplatform 12, rather than using the outputs 53 or 55, which respectivelyrepresent a direct measurement of target surface position and targetenergy consumption. Whatever method of measurement is used, someconversion factor, table, algorithm or other information base ispreferably stored in a digital memory so that a computer, controller oroperator initiated action can convert the measurement into the correctadjustment to be made to the target-to-substrate spacing that willoptimize coating uniformity on the substrate. Thus, using such wafercount alone, sufficient information, such as a table derived fromimperically collected or computer modeled conversion data, would bestored in the processor of the controller 50 to make possibledetermination of how much positional change of wafer 16 corresponds toeach wafer count. The conversion data so acquired would account for theeffects of all of the factors pecular to the specific target, cathodeassembly and sputtering apparatus, and process parameters employed.

From the above, it will be apparent to those of ordinary skill in theart that changes and additions can be made to the embodiments describedand above and illustrated in the drawings without departing from theprinciples of the present invention.

Therefore, what is claimed is:
 1. A method of maintaining, fromsubstrate to substrate, the uniformity, across the surface of asubstrate, of a film sputtered from a relatively thick sputtering targetthat substantially erodes over its life, the method comprising the stepof: sputtering, from a sputtering target of a given design, a film ontoa plurality of substrates at each of a plurality of distances from thetarget and measuring the film thickness uniformity across the surfacesof the substrates; based on film thickness uniformity measurements,empirically deriving target-to-substrate spacing as a function of targeterosion that will cause a film of a given uniformity to be sputteredonto a substrate from a target of the given design; sputtering, from asputtering target of the given design, a film of the given thicknessuniformity across the surface of a first substrate; then determining thestate of erosion of the sputtering target; then changingtarget-to-substrate spacing in accordance with the empirically derivedfunction; then sputtering a film of the given uniformity across thesurface of a second substrate from the sputtering target.
 2. The methodof claim 1 wherein: the step of determining the state of target erosionincludes the step of generating a signal representative of the state oferosion of the sputtering target; then the step of changingtarget-to-substrate spacing in accordance with the function of targeterosion is performed in response to the generated signal.
 3. The methodof claim 2 wherein: repeating the generating and changing steps; andthen sputtering a film of the given thickness uniformity across thesurface of a third substrate from the sputtering target.
 4. The methodof claim 2 further comprising the step of: sensing a target parameterrelated to the target erosion and generating the signal in responsethereto; calculating, in response to the signal, with a processor of acontroller, an adjusted target-to-substrate spacing and that willmaintain the given film thickness uniformity across the surface of thenext substrate to be sputter coated; and the spacing changing stepcomprising changing the target-to-substrate spacing to the calculatedadjusted target-to-substrate spacing.
 5. The method of claim 4 wherein:the sensing step comprises sensing the position of at least one point onthe surface of the target after erosion thereof; and the spacingchanging step comprises changing the spacing as a function of the sensedpositions on the surface of the target after erosion.
 6. The method ofclaim 4 wherein: the sensing step comprises measuring energy consumed bythe target from a beginning point in the life of the target; and thespacing changing step comprises changing the spacing as a function ofthe measured energy.